This invention relates to power transmission, and more particularly, to engine driven power transmissions having means for controlling the overall torque ratio of the transmission.
Specifically, this invention relates to engine driven multi-speed power transmissions having an added two-speed planetary for extending the torque range of the transmission.
When an underdrive ratio is added to a multi-speed planetary gear arrangement to provide increased output torque in the low range, particularly in a transverse front wheel drive, the underdrive is usually added to the output of the transmission so that the increased torque is not transmitted by the other gear members of the transmission. This approach is used so that the gear, shafts, etc., do not need to be redesigned for the increased torque. This results in an increased overall length for the transmission. Thus, the space required for the transmission is increased.
When the length of the transmission is critical, the alternative has been to place the underdrive set between the engine and transmission on the engine axis. However, this requires that power transmitting elements, such as gears and shafts, etc., of the main transmission be increased in size to accommodate the maximum torque that can be transmitted.
The maximum torque which can be delivered to the drive axle is the torque required to slip the drive wheels plus the brake torque. This maximum torque condition occurs when the vehicle is stationary with the engine throttle substantially fully opened to demand maximum engine torque and the operator has the brakes applied to prevent wheel slip. Under this condition, the torque converter is operating at the stall point which is the maximum torque ratio operating condition. Thus, it should be appreciated that significant torque levels can be attained in the transmission.
However, torque levels that will cause the tires to slip, in the absence of brake forces, are not useful in propelling the vehicle. Accordingly, the transmission gears, shafts, etc., are sized significantly greater than what is useful in order to accommodate a rarely achieved maximum torque conditioning.